This invention relates to the thread-forming fastener arts and more specifically to thread-forming fasteners for use with thermoplastics.
Thermoplastics are extremely important in manufacturing because of this relatively low cost, high degree of formability, corrosion resistance, and ease of conversion from raw material to final finished product. Thermoplastics are useful in applications including automotive, medical, recreational and disposable products. New compositions of thermoplastics are constantly being developed including materials which are reinforced with glass, carbon or other strengthening fibers. Often it is desirable to attach other structures to a body formed of thermoplastic or to attach the thermoplastic body itself to another structure.
When installing thread forming fasteners to thermoplastic materials, inherent problems of the thermoplastic materials are encountered, including a relatively high drive torque required to drive such fasteners combined with a low holding torque upon installation in such materials as well as the notch sensitivity of such materials at ambient temperatures. The combination of high drive torque and notch sensitivity results in thermoplastic materials being highly susceptible to crack formation upon the installation of thread forming fasteners. Such cracking is extremely prevalent when fastener installation is required in thin wall sections of thermoplastic material where material thickness and notch sensitivity combined with a high drive torque results in crack propagation.
Attempts have been made to overcome the cracking problems and/or subsequent loosening resulting under high drive torque, low holding torque and notch sensitivity. One solution to the problem of crack propogation and thin wall sections was to apply various sealants and/or adhesives. However, many times an adhesive does not prove sufficiently reliable and may not be able to tolerate either tensile or torsional forces. Further, such measures require additional materials as well as added time and labor in installation. In cost effective automotive or other assembly operations, time and labor must be optimized, and therefore such additional steps are undesirable.
Another attempt to overcome the problems of utilizing thread-forming fasteners with thermoplastics employed drilling and threading or tapping each individual hole into which a fastener was to be applied. However, such procedures are extremely expensive due to the additional time and tooling required for this additional tapping operation. Further, since the thread-tapping operation is very similar to a thread forming fastener being driven into a pilot hole, crack propagation and attendant problems may still occur.
Yet another attempt to overcome the problem of fastener attachment to thermoplastics was to mold fastener inserts or nuts directly into the thermoplastics body. However, this solution is impractical in many applications because of assembly requirements which are not conducive to preassembly of inserts, or because of the size or type of part to be manufactured, or the necessity to specifically locate each fastener, depending on the specific part, assembly and application. Further, even if the application was conducive to molding inserts into the thermoplastic body, such an operation can be overly costly. Molding inserts into thermoplastic requires additional set up and worker time. Additionally, such operations require special equipment and tooling to accommodate the inserts. Such measures may not prove cost effective.
Finally, even if the fasteners were applied to a drilled and tapped thermoplastic body or to inserts molded into the body, a fastener may to vibrate loose from the thermoplastic body since these methods may not reliably securely retain the fastener within the thermoplastic body.
Therefore, it is highly desirable to provide a fastener and method for applying such a fastener to a thermoplastic body which will not promote crack, propagation and preferably will reduce drive torque in applying such a fastener to a thermoplastic body and yet increase holding torque following installation. Heretofore, it is believed that no prior art device or method has resolved the above-noted problems associated with thermoplastics.